i88 UNSEGMENTED " WORMS" 



sporocyst, and migrate into the liver or some other organ. 

 Each sporocyst usually forms at a time 58 rediae ; each 

 of these forms 8-12 more rediae; and each of these forms 

 14-20 cercariae. In the winter a sporocyst may give rise 

 to cercariae directly. A redia is a cylindrical organism 

 with a short alimentary canal, excretory canals with " flame 

 cells," and a pair of blunt locomotor processes posteriorly. 

 A cercaria has a bifurcated gut, two suckers, a locomotor 

 tail, and the beginnings of gonads (Fig. 97 (6)). 



The cercariae emerge from the rediae, wriggle out of the 

 snail, pass into the water, and after swimming for a short 

 time, moor themselves to stems of damp grass. There 

 they lose their tails and become encysted. If the encysted 

 cercaria on the grass stem be eaten by a sheep, the cyst 

 is dissolved in the stomach, and the young fluke makes 

 its way up the bile duct and its tributaries. In about six 

 weeks it grows into the adult sexual fluke. 



It will be noted that the sporocyst is the modified embryo, but that 

 it has the power of giving rise asexually to redise. These develop, 

 however, from special cells of the sporocyst, which we may compare to 

 spores or to precociously developed parthenogenetic ova. Though the 

 reproduction is asexual, it is not comparable to budding or division. 

 The same power is possessed by the rediae, and there are thus several 

 (at least two) asexual generations between the embryo and the 

 adult. 



The disease of liver-rot in sheep is common and disastrous. It 

 has been known to destroy a million sheep in one year in Britain 

 alone. 



Classification. Order I. Heterocotylea, with a posterior ad- 

 hesive organ, often with a pair of accessory suckers beside the mouth. 

 Most are ectoparasitic. The development is direct and associated with 

 one host (monogenetic). 



e.g. Polystomum integerrimum, with many posterior suckers, often 

 in the bladder of the frog. It attaches itself in its youth to 

 the gills of tadpoles, passes thence through the food canal 

 to the bladder. 



Gyrodactylus, on the gills and fins of fresh-water fishes. It is 

 viviparous, but the embryo, before it is extruded, itself 

 contains an embryo, and this in turn another. 

 Diplozoon paradoxum consists of two individuals united. 

 The single embryo (Diporpa) is at first free-swimming, 

 but becomes a parasite on the gills of a minnow, 

 and there two individuals unite very closely and per- 

 manently. 

 Tristomuniy with three suckers, on some marine fishes. 



